Generally, as exemplified in FIG. 1, a charged particle (helium nucleus or electron) 8 constituting alpha particles or beta particles and passing through a substance 6 loses its energy (ΔE) in the substance 6 due to interaction with the substance 6. The loss ΔE is proportional to a type, a density and a thickness (t) of the substance.
On the other hand, the use of radioisotope sources has been increasing recently for calibration of radiation detectors and biological experiments, etc., in the fields of science, biology, chemistry, medical science and others. Further, based on the results of these studies and experiments, comparisons of other radiation doses and energies are performed. Thus, an energy, a dose or the like of a particle emitted from a radioisotope (hereinafter, simply referred to as an isotope) needs to be estimated accurately.
A radioisotope source (for example, 137Cs, 207Bi, 109Cd, 110mAg, 90Sr, etc.) emitting charged particles such as monoenergetic internal conversion electrons, beta particles, and the like is covered with a film in order to protect an isotope from external injury. Further, a thin film has been used to reduce an energy deposit of the charged particles in the film. Thus, a variety of studies and experiments have been conducted based on the assumption that the energy deposit in the thin film can be disregarded.
As an example of a thin film source 10, a 137Cs thin film source is shown in FIG. 2. 12 denotes an isotope composed of, for example, 137Cs and 14 denotes a thin film composed of, for example, aluminum in FIG. 2.
Conventionally, as shown in FIG. 3, measurements have been performed, for example, by a radiation detector 20 constituted by a scintillation detector (a detector composed of a scintillator and a photomultiplier device), a semiconductor detector, a gas detector, etc., regarding a hundred percent energy E as having been emitted from the source 10 without any loss.
However, results of a study by the present inventor have revealed that a charged particle 8 in fact lost an energy ΔE in the source 10 before getting out of the source 10, in accordance with its occurrence location 13 and emission direction, as shown in FIG. 4 and FIG. 5.
Conventionally, various efforts such as adjusting a radiation rate of radiation from a radioisotope source as described in Japanese Published Unexamined Patent Application No. 2004-221082, alleviating an influence of source fluctuations as described in Japanese Published Unexamined Patent Application No. 2006-275664 and measuring a radiation dose of a measuring object with accuracy as described in Japanese Published Unexamined Patent Application No. 2007-263804 have been made. However, an energy deposit within a radioisotope source has not received attention.
On the other hand, A. Martin Sanchez, et al., “An experimental study of symmetric and asymmetric peak-fitting parameters for alpha-particle spectrometry” Nuclear instruments and Methods in Physics Research A 339 (1994) 127-130 (hereinafter, referred to as literature 1) states that attention is given to an energy deposit within a radioisotope source. However, in a frequency distribution chart of energy intensity and frequency of counts of a particle group emitted from a radioisotope source (hereinafter, referred to as an energy distribution where its x-axis indicates energy intensity and its y-axis, frequency of counts) as shown in FIG. 6, a distribution (L1) based on an energy deposit of the particle within the radioisotope source 10 and a statistical fluctuation (L2) of the detector 20 are both treated as being symmetrical. As a result, an asymmetric energy spectrum (L3) obtained by an operation processing part 30 in actual measurement was not able to be expressed only by synthesizing the symmetric L1 and L2. Thus, L3 was reproduced as an asymmetric energy spectrum by adding an exponential function to the synthesis of the energy spectra of L1 and L2. However, there was no physical basis for the exponential function at all, and only an approximate estimation with the spectrum reproduced was conducted. Accordingly, an accurate estimation was not achieved. There are four serious mistakes in the method, including (1) the energy deposit of the particle within the radioisotope source is treated as being symmetric, (2) in spite of the fact that the particle loses its energy within the radioisotope source, an energy of the particle actually emitted outside the radioisotope source is treated as being equal to the initial energy which the particle possesses at the time of generation, (3) the exponential function having no physical basis is introduced only for forcibly expressing the asymmetry and (4) the performance of the radiation detector is not estimated with accuracy due to the introduction of the exponential function.
Further, in a conventional analytical method as in M. Miyajima, et al., “Numbers of scintillation photons produced in NaI (Tl) and plastic scintillator by gamma-rays.”, Published in IEEE Trans. Nucl. Sci. 40: 417-423, 1993 (hereinafter, referred to as literature 2) for example, an influence of the energy deposit within the radioisotope source was not estimated, and accordingly, energy calibration of the detector is incorrect. It can be found from an energy spectrum of a 976 keV internal conversion electron emitted from a 207Bi radioisotope source, having been measured by a radiation detector (plastic scintillator) as shown in FIG. 5 of the aforementioned literature 2 that an energy distribution of the 976 keV internal conversion electron having been emitted from the 207Bi radioisotope source is treated as being symmetric. As a result, the performance of the radiation detector was also estimated low.
Moreover, the internal conversion electron is treated as having only one level of energy without estimating internal conversion electrons having several different levels of energy (internal conversion electrons from K shell, L1 shell, L2 shell, L3 shell, M shell, etc.) in terms of excitation level of one nucleus. Thus, it is understood that the performance of the radiation detector is estimated lower.
Further, separation of ‘alpha particles’ having several different levels of energy is performed based on a result obtained by measurement in C. John Bland et al., “An Observed Correlation between Alpha-Particle Peak-fitting Parameters”, vol. 43, No. 1/2, pp. 223-227, 1992 (hereinafter, referred to as literature 3), G. Bortels et al., “ANALYTICAL FUNCTION FOR FITTING PEAKS IN ALPHA-PARTICLE SPECTRA FROM Si DETECTORS”, Applied Radiation and Isotopes, vol. 38, no. 10, pp. 831-837, 1987 (hereinafter, referred to as literature 4) and C. John BLAND et al., “Deconvolution of Alpha-Particle Spectra to Obtain Plutonium Isotopic Ratios”, Applied Radiation and Isotopes, vol. 43, no. 1/2, pp. 201-209, 1992 (hereinafter, referred to as literature 5). Separation is possible only because the measurement result of the alpha particles is matched with an approximate formula using the exponential function. For beta particles or gamma rays, for example, a result obtained by measurement cannot be expressed by an exponential function, and accordingly separation between different particles is impossible. From around 1970 until now, a great number of papers on and techniques about separating alpha particles as described above have been reported around the world. However, there have been no reports of any document ever estimating a type of particle. This is because they are not applicable to a particle other than alpha particles. Further, although it is said that the separation of alpha particles having different levels of energy is possible, there is a disadvantage that errors are large and measurement accuracy is remarkably low since approximation having no physical basis is repeated relative to a plurality of alpha particles.